An increase of the functional reliability of the hydraulic circuit of a hydraulically operable clutch generally aims at maintaining the existing shifting state of the clutch and obtaining between the shifting states a defined transition characteristic which can be reproduced and thus effectively controlled by the driver of the vehicle concerned.
To meet the requirements, pressure intensifiers are known from the prior art from which a sufficient feedback goes to the clutch pedal so that the engagement and disengagement ratios sought by the driver can be adapted to the existing situation. It is very important here to implement play and wear compensation in order to ensure a coincident response characteristic during the service life of the clutch.
Release cylinders thus are known from the prior art in which automatic wear compensation is implemented. In that case, the release cylinder can have two pistons; the axial spacing between which is determined by the volume of a liquid located between the pistons; there resulting an adaptation of the volume depending on the wear state of the clutch.
However, when the vehicle is stopped, there is the danger that in case the working valve is not sealed, hydraulic liquid flows into the intermediate space between primary and secondary pistons, via a bleeding hole and, due to the volume increase, moves the secondary piston so that a slow opening of the clutch cannot be ruled out with certainty whereby the vehicle, when the parking brake is open, could unintentionally set itself in motion.
In DE 197 17 486 C2 is described a pneumatic pressure intensifier for a hydraulic clutch operation for motor vehicles. In the pressure intensifier, a pneumatic working space is provided which can be loaded with pressurized air for pressure intensification of a hydraulic working space. One control piston, coordinated with the pneumatic working space and serving for engaging and disengaging the pressure intensification source, is activated by a working piston located in the hydraulic circuit so that a leakage, due to pressure increase in the release cylinder, does not occur.
However, the described pneumatic intensifier of the device has a complicated construction and, to that extent, is susceptible to noise and comparatively costly to produce.
Therefore, the invention is based on the problem of increasing the functional reliability of an operation in the hydraulic circuit of hydraulic clutch by comparatively simple means.